Changing Amp Valves

[Crazy_Joe]

I want to change my Laney's power amp valves from EL34's to 6L6's.
First of all how do i do it?
Secondly are these good valves to do it with: http://www.tube-shop.com/uk/prod_svetlana_detail_set.asp?ProductID=86&ProcessType=8

[shaman]

take it to a tech-biasing an amp can be dangerous stuff!!

[HTH AMPS]

6L6s can't handle as much voltage as EL34s so it depends on the plate voltage in your amp.  Most modern amps run around 450v to comply with EU regs, so you should be fine with the 500v max plate rating of 6L6GCs.

There is also the issue of the OT's primary impedance - EL34s like a different impedance to 6L6s for best power transfer/tone.

And lastly, 6L6s only pull 900mA per valve compared to 1.5A with EL34s - this means that your PT will not be loaded down as much and the heater voltage and plate voltage will rise.  'How Much' is the magic question, you'd have to take it to a tech and see how they work out.

In practice you'll most likely have no problems, I'm just making you aware of some considerations from a tech viewpoint.

Why do you want to change to 6L6s anyway?

 :twisted:

[Twinfan]

Joe - is this on a GH50L?  If so, there's a bias switch on the back so it's easy.

* Let amp cool
* Slowly and carefully remove EL34s by pulling them out with a slight wiggle
* Use a cloth so your hands don't touch the glass on the tubes
* Insert 6L6s slowly and carefully (using a cloth again), ensuring the pins and locating 'blob' line up properly
* Flick bias switch to 6L6
* Fire up and rock out

:twisted:

[Crazy_Joe]

Some guy on the Laney forum said you still have to bias it properly and it's not as easy as flicking the switch on the back.

[Twinfan]

Why have the switch then   :roll:

[Crazy_Joe]

I don't know i have no idea who is right :lol: but he made 2 massive stickies on the forum about amp biasing and stuff, _Tom_ thinks that the switch is just to get it in the general ballpark but he still thinks you need to bias it properly.

[WezV]

there is a hell of a lot of variation between valves of the same type so an amp would still need re-biasing.  Quality control on these things is awfull

However you can buy groove tubes pre-tested and rated valves.  Once you have had your amp set up and rebiased for a certain perfornmance level of valve you can just buy the same rated set from them each time and probably wont need to re-bias.  

I imagine that the switch on the back of the amp is to get it to the levels where the different valve type wont endanger the performance of the amp, but that doesnt mean the valve will automatically be running at the right temperature and a bit of re-biasing may be in order to achive the best tone

[Crazy_Joe]

I don't know of any amp techs around here. :(

[WezV]

time to find one then - there will be one somewhere.  

If you want valve amps a good amp tech is a must - these things need a bit more care and maintenance than we are used to having to do in modern life.

Or buy a simple 5w valve amp that you can happily pull out and replace the valves on without worrying about bias - just keep trying valves till you find one that sounds good

[HTH AMPS]

I didn't realise that amp had a switch to go between EL34s and 6L6s - as previously indicated, this will just be a 'ball-park' setting so that most EL34s or 6L6s will work without running hot.

I would still check the bias though - all the switch does is select two different resistors in the bias circuit to set the adjustable bias range.

I'd be interested to see how Laney addressed the OT issue since EL34s and 6L6s want to see a different primary Z.   I'd guess they geared the amp towards EL34s and used the 6L6 setting as an extra (but not optimal)setting.

 :twisted:

[steve]

Why have the switch then   :roll:

The switch is there so you can set the bias between the two different valves, it does not mean you can just drop them in and away you go. It will still need a bias. I have the same set up on my Speed Twin head.